Automotive corrosion resistant steel sheets commercially used today include electrogalvanized steel sheets, steel sheets with electroplated Zn--Ni alloys, steel sheets with electroplated Zn--Fe alloys, hot-dip galvannealed steel sheets and various other types, all of which are Zn base plated steel sheets. These make use of the self-sacrificial corrosion preventing action of Zn for steels. The most straightforward way to improve corrosion resistance is by increasing the coating weight of plating (hereunder referred to as "coating weight") but the increase in coating weight is accompanied by deterioration in formability, weldability and other quality factors.
Attempts have therefore been made to alloy Zn with other elements so that smaller coating weights than that of pure Zn will suffice for providing comparable degree of corrosion resistance. Potential effects of alloying include, for example, bringing the corrosion potential of the alloy even closer to steel so that the corrosion rate of the plating layer per se is allowed down, and stabilizing the corrosion product. However, the contribution of alloying to the improvement in corrosion resistance has been still unsatisfactory in the conventional Zn base alloy plated steel sheets. Under the circumstances, attempts have been made in recent years to add Cr as an alloying element to the Zn base plating layer. Examples of such attempts have been proposed in Japanese Patent Application (kokai) Nos. Hei 1-191797, 3-120393, etc. It is true that as far as the corrosion resistance in the bare state is concerned, increasing the percent Cr content contributes to the formation of a Zn--Cr alloy plating that exhibits better corrosion resistance than the conventional Zn base alloy plating.
As an example, a salt spray test was conducted in accordance with JIS Z 2371 and the number of days to 2% red rust development was checked. The results are shown in FIG. 1. Motorcar bodies are normally formed before use, so the test specimens were those which had been subjected to 17% stretch. In the following description, values of coating weight are sometimes indicated with the symbol for unit of its measure (g/m.sup.2) being omitted. For example, a coating weight of 30 g/m.sup.2 may be indicated as coating weight 30. In FIG. 1, EG 30 designates a commercial electrogalvanized steel sheet with coating weight 30; GA 60 is a commercial hot-dip galvannealed steel sheet with coating weight 60; and Zn--Ni 30 designates a commercial Zn--Ni alloy plated steel sheet with coating weight 30 and 13% Ni content. For all Zn--Cr specimens, the coating weight of the plating was 20 g/m.sup.2.
One can see from FIG. 1 that the corrosion resistance of the Zn--Cr alloy plated steel sheet in the bare state improves almost linearly with the increase in the percent Cr content of the alloy. It can also be seen that even with coating weight 20, the samples have better corrosion resistance in the bare state than EG 30 and GA 60 of higher coating weight if Cr/(Cr+Zn) is 2 wt % or more. Thus, the Zn--Cr alloy plated steel sheet exhibits better corrosion resistance in the bare state and this would be because in a corrosive environment, the surface oxide film of Cr suppresses the dissolved oxygen reducing reaction by a marked degree to reduce the corrosion current density, or retard the corrosion rate.
The experimental result under consideration is that of a test assuming corrosion that occurs principally in a site such as where the inner surface of an automotive body is electrodeposited with so small coatings that the surface is partially left in the bare state. Speaking of the corrosion resistance of various surface treated steel sheets, it is largely dependent on the nature of corrosive environment and the ranking in corrosion resistance can vary as a result of the change in the environment. In recent years, as the sophistication of car models has become an industrial trend, there is a growing rigor in the demand for the corrosion resistance against rust that will develop on the exterior surfaces of automotive bodies. Cosmetic corrosion progresses under coatings starting at the damaged site of the coating due primarily to such factors as the throwing of pebbles by the wheel of a running vehicle and it will impair the vehicle's external appearance in the form of red rust, blistering of coatings or the like.
As already mentioned, the corrosion resistance of the Zn--Cr alloy plated steel sheet improves linearly with the increase in the percent Cr content in the corrosive environment on the inner surface of an automotive body. In contrast, the resistance against rusting on the exterior surface of an automotive body will not necessarily improve in response to the increase in the percent Cr content but may occasionally deteriorate in response to the increase in the percent Cr content. Hence, the Zn--Cr alloy plated steel sheet has had the problem that compared to other Zn base plated steel sheets, its corrosion resistance in the bare state is good but the resistance against rusting on the exterior surface of an automotive body (cosmetic corrosion) is poor.
Therefore, the first object of the present invention is to provide a corrosion resistant steel sheet that is improved not only in corrosion resistance but also in resistance against cosmetic corrosion.
While the improvement in corrosion resistance by alloying has been described above, it should of course be understood the coating weight also presents a significant effect.
As an example, the result of the test assuming corrosion that occurs in the case of use on the exterior surface of an automotive body is shown in FIG. 2. Since the exterior surface of an automotive body is usually provided with coatings, corrosion starts at the damaged site of the coating due to such factors as the throwing of pebbles by the wheel of an automobile. Corrosion resistance tests on motorcar bodies can most reliably be performed with actual car models. However, on account of the longevity of time that passes before the result of evaluation becomes available and due to the cost problem, the methods commonly employed include the exposure to atmospheric air of coated test specimens that in which specified scribes have been made, and the use of a cyclic corrosion tester that creates artificially an accelerated corrosion environment by the appropriate combination of salt spray with drying and humidifying cycles. The data in FIG. 2 show the results of measurement for the blister width of coatings that was conducted after performing a cyclic corrosion test (for the test cycles, see FIG. 3) for 2 months on steel samples that had been subjected to chemical conversion treatment with zinc phosphate and 3-coat application, followed by scribing to the substrate steel.
Indicated by "pure Zn" in FIG. 2 is a galvanized steel sheet that was prepared by an electrogalvanization technique in the usual manner (which is hereunder designated as "EG"). "GA" refers to a commercial hot-dip galvannealed steel sheet. "Zn-13 wt % Ni" refers to a commercial Zn--Ni alloy plated steel sheet with 13 wt % Ni content (which is hereunder designated as "Zn--Ni"). "Zn-13 wt % Cr" refers to a Zn--Cr alloy plated steel sheet with 13 wt % Cr content (which is hereunder referred to as "Zn--Cr"). As one can see from FIG. 2, all alloy plated steel sheets tested were improved in corrosion resistance compared to EG with the same level of coating weight but the alloying effect was the greatest in the Zn--Cr alloy plated steel sheet.
However, the coating weight also presents a significant effect and, hence, the Zn--Cr alloy plated steel sample with coating weight 10 is superior to EG with coating weight 20 but inferior to EG or Zn--Ni alloy plated steel sample with coating weight 30. Further, in order to insure comparable corrosion resistance to that of the hot-dip galvannealed steel sheet with a coating weight of 60 g/m.sup.2 which is domestically used today in the largest quantity, even the Zn--Cr alloyed plated steel requires 30 g/m.sup.2. Thus, any plating species provides better corrosion resistance as the coating weight increases and the change is particularly marked when the coating weight is in the range from 10 to 30 g/m.sup.2. However, especially in the case of the Zn--Cr alloy plated steel sheet, the formability deteriorates sharply in response to the increase in coating weight and, hence, it has suffered from the problem of low practical feasibility due to poor formability in spite of its high corrosion resistance.
Therefore, the second object of the present invention is to provide a corrosion resistant steel sheet that is improved not only in corrosion resistance but also in formability.
As already pointed out, the recent industrial trend for the sophistication of car models has created a growing rigor in the demand for the corrosion resistance against rust that will develop on the exterior surfaces of automotive bodies. Cosmetic corrosion progresses under coatings starting at the damaged site of the coating due primarily to such factors as the throwing of pebbles by the wheel of a running vehicle (which are hereunder collectively designated as "chipping") and it will impair the vehicle's external appearance in the form of red rust, blistering of coatings or the like. Therefore, endurance against chipping which triggers corrosion is an important factor to be considered.
In a corrosive environment on the inner surface, the corrosion resistance of the Zn--Cr alloy plated steel sheet improves linearly in response to the increase in the percent Cr content. However, the resistance to chipping does not improve necessarily in response to the increase in the percent Cr content; to the contrary, the chipping resistance tends to deteriorate in response to the increasing percent Cr content. Hence, the Zn--Cr alloy plated steel sheet has had the problem that compared to other Zn base plated steel sheets, its corrosion resistance in the bare state is good but the chipping resistance is poor.
Therefore, the third object of the present invention is to provide a corrosion resistant steel sheet that is improved in chipping resistance.
Further, if one wants to form the Zn--Cr plating and yet achieve as strong corrosion resistance as before it is formed, he may increase either the Cr content or the coating weight. However, the approach of increasing the Cr content is limited in effectiveness since if it exceeds 30 wt %, the adhesion of the plating perse will deteriorate. The approach of increasing the coating weight is also inappropriate since this will cause the same type of deterioration in quality as in the aforementioned case of the prior art Zn plating.
Therefore, the fourth object of the present invention is to provide a corrosion resistant steel sheet that is improved not only in corrosion resistance before forming but also in corrosion resistance after forming.
In the current production line of automotive bodies, paints are applied over platings and, hence, the adhesion of coatings is also an important factor. An example of this practice is the chemical conversion treatment with zinc phosphate, followed by three-coat application comprising cationic electrodeposition coating, intermediate coating and top coating. To evaluate the adhesion of the applied coats, the sample formed by this method was sealed on both the back surface and the end faces, immersed in pure water at 50.degree. C. for 10 days, recovered from the water and immediately subjected to a cross cut adhesion test. The result of visual check on this test sample is shown in FIG. 4. For comparison, the results on conventional Zn platings are also shown in FIG. 4. As one can see from the figure, the Zn--Cr alloy plating is inferior to the conventional Zn plating in terms of water resistant secondary adherence of coating.
Therefore, the fifth object of the present invention is to provide a corrosion resistant steel sheet that is improved not only in corrosion resistance but also in water resistant secondary adherence of coating.
The foregoing experimental results relate to corrosion resistance in the bare state. In the current production line of automotive bodies, the chemical conversion treatment is followed by cationic electrodeposition coating and, on the exterior surfaces of car bodies, intermediate and top coatings are applied to produce a total of three coats; however, the inner surfaces are generally used with the electrodeposited coat alone. On the inner surfaces, a certain type of corrosion may occasionally become a problem in that corrosion as it started from areas of low throwing power in electrodeposition coating, such as those around mating surfaces including hem-flange Of door, progress under the coating to eventually result in perforation. If this problem is a real concern, the corrosion resistance of the plating layer perse is not sufficient and a total corrosion inhibiting schedule is required taking into account the combination with the coatings. As already mentioned, the corrosion resistance of the Zn--Cr alloy plated steel sheet in the bare state improves linearly with the increase in the percent Cr content; however, after electrodeposition coating, perforation corrosion tends to progress as a function of the increase in the percent Cr content. Hence, the Zn--Cr alloy plated steel sheet which has better corrosion resistance in the bare state than other Zn base plated steel sheets has suffered from the problem of lower resistance to perforation.
Therefore, the sixth object of the present invention is to provide a corrosion resistant steel sheet that has improved perforation corrosion resistance.